Wind turbines are equipped with a drive train comprising a wind power rotor and an electric generator, which are connected together by a gearbox that delivers the torque from the turning rotor to the generator, which in turn creates electricity.
High power capacity wind turbines, i.e., machines with a rated power exceeding 4.5 MW, employ planetary-type gearboxes with at least two mixed planetary stages, gearboxes with parallel stages or even gearboxes with mixed stages. In other words, they are equipped with both planetary and parallel sorts, which require, regardless of the type of gearbox harnessed, lubrication systems with their corresponding oil baths for the different gearboxes stages. These lubrication systems are usually controlled by mechanical or electrical pumps, or by solenoid valves to guarantee that the oil remains at a sufficient level so that the parts of the gearbox with metal contacts are lubricated at all times.
During a wind turbine's lifecycle, there are long periods without connection to the power grid, viz., when erecting wind turbines in the wind farm, during maintenance tasks or whenever the electrical grid has failed. Consequently, during these periods, the wind turbine's lubrication systems stop functioning. The wind turbine, with the blades feathered, nevertheless has a certain rotation; hence its metal parts in contact must likewise be kept lubricated during this period of inactivity.
The gearbox oil could thus empty into an external tank for various reasons. During the periods in which the machine is active, the lubrication system pumps the oil into the external tank to maintain an adequate level for filtering and cooling the oil, returning it to the gearbox once these tasks have been concluded.
During periods of wind turbine inactivity, and given that the nacelle rests at a 6° angle with respect to the standing tower, set so with a view to preventing potential impact of the blades against the tower base during deflection generated under the force of the wind, the oil begins to drain into the external tank caused by the mere force of gravity at this angle. The foregoing, added to the splashes provoked by unloaded rotating gears and nonoperational lubrication systems, oil consequently does not return to the gearbox, causing the oil level of the baths in the various gearbox stages to descend, and resulting in the lack of lubrication for the different metal parts in contact.